1. Field of the Invention
This invention relates generally to methods of surface treatment of ferrous materials, commonly referred to as forging billet stock, and particularly, relates to methods for surface treatment of such stock to inhibit the formation thereon of oxides during heating for forging.
2. Description of the Prior Art
Many products, including hand tools, such as wrenches and the like, are formed by forging. In this process, a hot-rolled bar formed of a suitable ferrous alloy, such as a suitable steel, which bar constitute forging billet stock, is heated to a forging temperature, typically in the range of from about 1500.degree. F. to about 2300.degree. F., and forged in forging dies. Then, the forged billet passes through a trim station to remove excess material, thereby forming the forged part or "forging." The forged part then typically undergoes a number of finishing processes to produce the desired quality of surface finish on the part. It is important that the surface finish quality and consistency of the as-forged part be as good as possible to minimize the extent of post-forging finishing operations. To this end, trim punches are carefully designed to eliminate marks created during trimming and the forging dies may be polished to eliminate the transfer of machining marks on the dies to the forged part.
Typically, hot rolled billet stock, as received from the mill, has surface oxidation scale which can adversely affect the surface condition of the forged part. This scale can be removed by a number of known methods, including centerless grinding with abrasive wheels or belts, steel grit and steel shot blasting, glass beading, slag blasting, drawing or bending and machining. However, even though mill scale may be cleaned from the billet stock by these methods, the heating of the cleaned billet in air to a forging temperature results in the rapid growth of additional thick oxide surface layers, typically characterized by high variability of thickness and non-uniform adherence. These oxidation layers require additional processing to remove them from the forged part. Thus, the oxidation layers may be forged into the surface of the part, to varying depths, resulting in an uneven and inconsistent surface finish. In order to bring the surface of the forged part to a required quality and consistency, hand and machine polishing are necessary and may result in the removal of as much as 0.01 inch of stock per surface. This type of heavy stock removal is tantamount to regrinding and regenerating the entire shape of the part, altering the complex three-dimensional, as-forged geometry, thereby negating the painstakingly achieved precision geometry of the forging dies.
Furthermore, because of the variable adherence of the oxidation layers to the billet surface, some of the oxide layer will flake off. Some of these flakes may be impacted randomly into the forged part by the forging pressures, as described above, further aggravating the surface irregularity problem. Other scale residue may be left in the forging dies. Such residue can be driven into the next forging billet, creating surface irregularities therein. In order to avoid this accumulation of oxide scale in the forging dies, expensive automatic blow-off and vacuum collection systems have been necessitated.
Efforts have previously been made to inhibit the formation of oxide scale during the heating of forging billet stock to forging temperature by treatment of the billet stock prior to heating it to the forging temperature. Thus, commercially available scale preventive coatings, such as glass/alcohol coatings and graphite/water coatings, have been utilized on billet stock. However, such commercially available coatings have, generally, been found to be ineffective in preventing scale formation during heating for forging. Also, they are relatively difficult to apply to billet stock, require drying or curing after application, and can be relatively easily rubbed off by handling.